Chromium picolinate has the potential to cause DNA damage, leading to birth defects and cancer, in humans. I'm a chemistry professor and work just down the hall from one of the major metallobiochemistry groups that are studying this. (See, for example, this press release, which is about work published in the prestigious "Proceedings of the National Academy of Science".) This is real, carefully controlled peer-reviewed science funded by the National Institutes of Health and the American Diabetes Association.

The problem is not the chromium, it's the picolinate ligand. Picolinate is NOT naturally occurring. (In fact, it's a derivative of the explosive picolinic acid!) It's still not clear how helpful chromium supplements are in controlling blood sugar--that's an area that definitely deserves more research. (And, there are also on-going studies about this, again funded by NIH and ADA.) However, there are other forms of chromium supplements that should be safer than chromium picolinate. For example, chromium polynicotinate (aka GTF chromium) is not known to have the health risks of the picolinate ligand.